The present invention relates to frequency synthesizers and, more particularly, to a frequency synthesizer having high bandwidth, a low level of spurious frequencies and phase noise, and high resolution. Such a frequency synthesizer would have many applications in electronic communications, for example, for up-conversion and down-conversion.
High resolution variable frequency synthesizers are known. One class of frequency synthesizers is the class of direct digital synthesizers (DDS), for example the model Q2240 DDS available from QUALCOMM Inc. of San Diego Calif. DDSs have variable frequency output at very high resolution, but have relatively high levels of spurious output frequencies. DDSs typically produce frequencies in the MHz range, which is far below frequency bands, such as the L-band (0.950-1.535 GHz) that are of interest for communications. The frequency produced by a DDS may be multiplied, but this also multiplies the spurious frequencies.
FIG. 1A is a block diagram of a circuit recommended by QUALCOMM in their Synthesizer Products Data Book for boosting the frequency output by a DDS without a corresponding boost in the spurious frequencies. The output of a DDS 10 is converted to an analog DDS output signal (frequency=fDDS) by a digital-to-analog converter 12. The DDS output signal is filtered by a bandpass filter 14 and mixed with a mixing frequency signal, of mixing frequency fMIX greater than  greater than fDDS, in a mixer 16 to produce a signal that includes the sum and the difference of the frequencies of the two signals input to mixer 16. Mixing frequency fMIX also is called a local oscillator frequency fLO. The output of mixer 16 is divided by a divider 18 to reduce the frequency of the mixer output by a factor of P. The resulting divided signal is filtered by a second bandpass filter 20 to produce an output signal with the desired frequency ((fM+fDDS)/P or (fMxe2x88x92fDDS)/P). The division by P also reduces the noise (phase noise and spurious frequencies) in the input to mixer 16, so that the output of the circuit of FIG. 1A is much cleaner than the signal that would be obtained simply by multiplying the output of DDS 10.
DDSs have very high resolution but also have limited bandwidth. QUALCOMM recommends the circuit of FIG. 1B for increasing the bandwidth of a DDS. As in the circuit of FIG. 1A, the output of DDS 10 is converted to an analog DDS output signal (frequency=fDDS) by digital-to-analog converter 12 and is filtered by a bandpass filter 22. The filtered output is amplified by an amplifier 24. The frequency of the amplified signal is multiplied by a variable factor M in a multiplier 26 and further filtered by a second bandpass filter 28. The circuit of FIG. 1B has a much higher bandwidth than DDS 10, but is noisier than the circuit of FIG. 1A because of the multiplication in multiplier 26.
There is thus a widely recognized need for, and it would be highly advantageous to have, a frequency synthesizer that combines the high resolution of a DDS with high bandwidth and low noise.
According to the present invention there is provided a wide bandwidth, high resolution, clean frequency synthesizer including: (a) a direct digital synthesizer; (b) a digital-to-analog converter for receiving output from the direct digital synthesizer and producing a DDS output signal; (c) a first bandpass filter for filtering the DDS output signal; (d) a source of a mixing frequency signal; (e) a single side band mixer for mixing the mixing frequency signal with the filtered DDS output signal, thereby producing a mixer output signal; (f) a divider for dividing the mixer output signal, thereby producing a divided signal; and (g) a second bandpass filter for filtering the divided signal.
The present invention includes two innovations. The first innovation is the use of a single side band mixer instead of the mixer of FIG. 1A. The advantage of a single side band mixer is that a single side band mixer outputs either the sum or the difference of the input frequencies, but not both. The second innovation is following bandpass filter 20 with a phase lock loop (PLL). A typical PLL lacks the high resolution of a DDS, but has much cleaner output and a much wider bandwidth than a DDS, and reaches higher frequencies than a DDS.